1. Field of the Present Invention
The present invention relates to a lithographic apparatus, a device manufacturing method, and a device manufactured thereby.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatuses can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements, which serve to generate the circuit pattern. This is generally referred to as maskless lithography.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In maskless lithography a beam of radiation illuminates the array of individually controllable elements. Each element can be controlled by a separate control signal, such that each element can assume one of a number of states. A desired pattern can thus be imparted to the cross section of the beam, that pattern being dependent upon the state of each element within the array. In cross section in a target plane, the projected pattern can be regarded as comprising an array of radiation spots formed by an array of sub-beams, with each spot corresponding to, and controlled by, an individual element or group of elements within the array. The energy of each spot is thus dependent upon the state of the corresponding individually controllable element or elements.
The lithographic apparatus can comprise a projection system arranged to project the patterned beam onto the target plane. In normal operation of the lithographic apparatus (i.e., when it is being used to project a pattern onto a target portion of a substrate) a target surface of the wafer containing the target portion is arranged to be substantially coincident with the target plane and in the path of the patterned beam. Consequently, an array of spots is projected onto the target portion of the substrate, each spot corresponding to an individual sub-beam. Measurement of the received radiation energy for each spot provides an indication of the radiation energy for the corresponding sub-beam. The energy of each spot is ultimately controlled by the corresponding individual element (or group of corresponding individual elements) within the array. Therefore, measurement of the energy of a spot provides an indication of the state of the corresponding individually controllable element(s). Measurement of the energy of the spots can be used to calibrate the array of individually controllable elements.
Each element within the array can be controlled such that it is fully on, fully off, or in one of a number of intermediate states, such that substantially all, substantially none, or an intermediate amount of the radiation incident upon that element is reflected or transmitted towards the target plane. Consequently, the energy of each spot at the target plane can vary according to the state of the corresponding element or elements.
Over time the performance of the lithographic apparatus, in particular the array of individually controllable elements, can become degraded. For instance, this degradation can be caused by contamination. Contamination can affect the performance of some elements more than others. Additionally, individual elements within the array can malfunction. The ability of each element to accurately change its state can vary over time. Consequently, the power of the radiation within each sub-beam may not accurately reflect the control signal supplied to the individually controllable element. It is desirable to be able to calibrate the array of individually controllable elements so as to detect and compensate for any malfunction or degradation.
One arrangement to calibrate the performance of the array of individually controllable elements could be to provide a dose sensor. A dose sensor can comprise a photodiode arranged to measure the radiation energy within a single spot illuminating the photodiode. Certain types of element malfunctions can result in the element being stuck in the fully on position (or an intermediate state) when the control signal indicates that it should be fully off. Therefore, it cannot be assumed that by providing the appropriate control signals to turn all but one element off, the only radiation received by the dose sensor will be from the remaining element. A single dose sensor may not be able to distinguish between radiation received from the element being measured and stray radiation received from other elements. Therefore, in order to exclude radiation from the other elements, a pinhole in a screen over the photodiode can be provided. The pinhole is sized such that only radiation from a single element can pass through.
In order to measure the energy of every spot the dose sensor can be aligned with each spot in turn. This provides considerable difficulties in aligning each spot accurately with the pinhole. Additionally, the sensor has to measure each spot for a minimum period of time in order to accurately measure its energy. Consequently, calibration of the whole array of individually controllable elements can be extremely time consuming.
With a dose sensor as described above, in order to calibrate the ability of each element at a range of element states then the energy of each spot must be measured separately a number of times. This further increases the amount of time required to calibrate the array.
Therefore, what is needed is a system and method that allow for a more effective determination for malfunction or degradation of an image spot.